1. Field of the Invention
The present invention relates to a power supply device and, more particularly, to a DC/DC power supply device which is capable of converting a D.C. input to a predetermined different D.C. output.
2. Description of the Related Art
Referring to FIG. 3 showing the circuit of a known power supply apparatus, a switching element 3 is connected to the primary winding of a high-frequency transformer 1. A power supply pulse width modulator integrated circuit 2 (abbreviated as "PWMIC" hereinafter) for controlling the switching operation of the switching element 3 is connected to the switching element 3. The switching element 3 and the PWMIC 2 in combination provides a switching device for switching the input electrical power. The PWMIC 2 is supplied with D.C. power connected between input terminals P and N through a rectifier circuit 30 which includes a current diode 4 and a smoothing capacitor 5.
Output terminals T1, T2, T3 and T4 are connected to the secondary windings W1, W2, W3 and W4 of the high-frequency transformer 1 through rectifier circuits 31, 32, 33 and 34, respectively. The rectifier circuit 31 includes a current diode 10 and a smoothing capacitor 13. Similarly, the rectifier circuits 32, 33 and 34 are composed of a current diode 16 and a smoothing capacitor 17, a current diode 18 and a smoothing capacitor 19, and a current diode 20 and a smoothing capacitor 21. Output voltages OUT1, OUT2, OUT3 and OUT4, which are insulated from one another, are delivered from these output terminals T1, T2, T3 and T4. Thus, the high-frequency transformer 1 and the rectifier circuits 31 to 34 in combination provide a power conversion means by which the high-frequency power converted from the D.C. input through the switching device 2, 3 is again converted into D.C. power.
A shunt regulator 12 and a resistor 8 are connected in series between the output terminals T1 for delivering the output power OUT 1. One resistor 14 of resistors 9 and 14 for dividing the voltage between the output terminals T1 is connected to the shunt regulator 12. A phase correction capacitor 11 is connected between the point where the shunt regulator 12 and the resistor 8 are connected and the point at which the voltage dividing resistors 9 and 14 are connected to each other. A photo-coupler 6 and a resistor 7 are connected in series across the resistor 8. The photo-coupler 6 also is connected between the F/B terminal of the PWMIC 2 and the switching element 3. The shunt regulator 12, photo-coupler 6, resistors 7 to 9 and 14, and the capacitor 11 cooperate to provide a voltage maintaining means which maintains a constant voltage between the output terminals T1 through a feedback to the PWMIC 2 and the switching element 3.
The operation of this power supply apparatus is as follows.
When a D.C. power signal of a predetermined level is supplied between the input terminals P and N, the PWMIC 2 is supplied with the power so as to start to control the switching operation of the switching element 3. In consequence, voltages are generated in the secondary windings W1 to W4 and these voltages converted into D.C. voltages by the respective rectifier circuits 31 to 34.
When a load is connected between the output terminals T1, load current flows through the winding W1 so that a voltage drop appears across the winding W1. In consequence, the voltage between the output terminals T1 varies according to the load current. The voltage between the output terminals T1 is divided by the resistors 9 and 14 and, if the voltage obtained through the division is higher than a reference voltage set by the shunt regulator 12, the shunt regulator 12 becomes conductive so that a feedback signal is sent to the PWMIC 2 and the switching element 3 through the photo-coupler 6. In consequence, both the switching frequency and the duty ratio of the switching element are reduced and the voltage generated in the winding W1 is lowered. Conversely, when the voltage produced as a result of the division is lower than the above-mentioned reference voltage, no feedback is made to the PWMIC 2 and the switching element 3. In this case, the switching operation is conducted at the switching frequency and the duty ratio which have been set in the PWMIC 2, with the result that the voltage generated in the winding W1 is raised.
It is therefore possible to maintain the voltage between the output terminals T1 at the desired level, by suitably selecting the resistance values of the resistors 7 and 14. The output voltage is maintained constant regardless of a change in the input D.C. voltage. Furthermore, since the voltage across the winding W1 is constant, other windings W2 to W4 also produce output power of constant voltage levels corresponding to the ratio of turns of windings.
Thus, in the known power supply apparatus, the output voltage in one output winding W1 of a plurality of output windings is fed back to the input switching device which is composed of the PWMIC 2 and the switching element 3, so as to change the switching frequency and the switching duty ratio, thereby preventing any change in the output voltage between the output terminals T1 attributable to the voltage drop across the winding W1 caused by the load current.
This known power supply apparatus suffers from a disadvantage in that, although the output voltage between the output terminals T1 is stabilized, the voltages generated in other windings W2, W3 and W4 are undesirably changed due to the changes in the switching frequency and the duty ratio in the switching device. Namely, the windings W2, W3 and W4 are directly influenced by the level of the load current flowing through the winding W1.